In this paper, a series of Er ∶ PbF2 crystals were successfully grown by Bridgman method. The first-principles calculation based on the density functional theory was applied to investigate the clustering effect of Er3 + in PbF2 crystals in details. The relationship between the up-conversion (UC) luminescence properties ( luminescence intensity, color variation) and the cluster structure in Er∶PbF2 crystals was obtained for the first time. It is found that as Er3 + concentration increases, the clusters evolve from monomers to higher-order configurations, with the distance between Er3 + ions decreasing and then increasing, resulting in the intensity of the red emission in the UC fluorescence first increasing and then decreasing, and the red-green luminescence ratio also decreases after the Er3 + concentration is higher than 6. 5% (mole fraction), indicating that the luminescence color can be adjusted from red to yellow-green. This study proves that the structural evolution of rare-earth ion clusters could regulate the spectroscopic properties of Er∶PbF2 , which provides a novel method for the design of multi-color luminescent materials.